The field of digital communication has grown significantly in the last decade due to recent advantages in wideband communication channels and solid-state electronics. Digital communication describes the transmission of a sequence of digital messages or a digitized analog signal.
Digital communication signals are used to send digital data from a transmitter to a receiver. The signals may be transmitted using wireless or wired media. The data may represent any type of information, such as text, sound, images, or computer files. Such information, which is unknown to the receiver until it has received and decoded the data, is defined herein as the “unknown portion” of the communication signal.
Communication signals are typically impaired by a channel before arriving at a receiver's input. In wireless systems, the signals often arrive at the receiver via multiple paths. This type of channel impairment is known as “multipath.” Due to the varying delays and attenuations among the multiple paths, the signals may add constructively or destructively. When the signals add destructively, it is commonly known as “fading,” as the combined signal becomes attenuated, or “fades.” Depending on the bandwidth of the signals and the delays of the multiple paths, different frequency components of the signal may experience different levels of fading. This is known as “frequency-selective” fading. The multiple signals arriving at a receiver are often described relative to a “main” or strongest path, combined with “echoes” or weaker paths. In broadcast television, the echoes are known as “ghosts” due to the perceived effect of multipath on analog TV signals. The delay between the earliest arriving path and the latest arriving path at the receiver is commonly referred to as the “delay spread” of the channel.
A typical digital communication system uses a sinusoidal carrier, whose amplitude and/or phase are modulated in order to communicate information. Although the nominal carrier frequency is known to both the transmitter and receiver, there is typically a frequency offset between the carrier generated by the transmitter and a locally generated carrier signal in the receiver. Therefore, a mechanism is typically provided in the receiver to recover the precise frequency and phase of the carrier generated by the transmitter. Such mechanisms are known as carrier recovery.
In a typical communication system, signals are transmitted according to a specified time base. Although the nominal time base is known to both transmitter and receiver, there is typically a timing offset between the time base used by the transmitter and the local time base in the receiver. Therefore, a mechanism is typically provided in the receiver to recover the precise timing of the transmitted signal. Such mechanisms are known as timing recovery.
To aid the receiver, many communication signals also include pre-defined signals, such as synchronization or framing signals. The receiver can be designed to utilize these signals for various purposes, including carrier recovery, timing recovery, frame alignment, and channel estimation. Such signals are defined herein as the known portion of the communication signal.
One digital communication system that defines such a known portion for its signal is the standard OB20600-2006 system (used for digital terrestrial television broadcast in China), also known as Digital Television Multimedia Broadcast (DTMB). It defines three different header modes, each using a pseudo-random number (PN) sequence to generate a known header signal for each transmitted frame. In one mode, the unknown portion of the signal is transmitted using orthogonal frequency domain modulation (OFDM) techniques. In that mode, the known PN sequence constitutes the guard interval (01) of the OFDM frame, and is an alternative to the cyclic prefix (CP) typically used in OFDM systems such as digital video broadcasting-terrestrial (DVB-T) used for digital television broadcasting in other parts of the world. The CP is a repeated portion of the unknown signal, so unlike the PN sequence used in DTMB, the receiver does not know it in advance.
In most DTMB receivers, the known PN sequences are used for channel estimation. Unlike most CP-based OFDM systems, the DTMB system does not include frequency domain pilot signals that can be used for channel estimation.
A conventional DTMB receiver includes a channel estimation block. The channel estimation block uses PN sequence correlation techniques to estimate the channel impulse response. The conventional DTMB receiver further includes an OFDM symbol restoration block, which performs two functions: 1) to remove the PN sequence from the guard interval, and 2) restoration of the channel spread OFDM symbols.
Conventional approaches to channel estimation using PN correlation are preconditioned on a cyclic form of the PN sequence. Therefore, this approach is not applicable for non-cyclic PN sequences (such as PN595 used in DTMB header mode 2). Further, such an approach is undesirable for cyclic (or quasi-cyclic) PN sequences, as channel estimation using PN correlation is quite complex when the channel delay spread is longer than the cyclic PN period. Although, an iterative interference cancellation technique has been developed to handle long channel delay spreads, this technique is impractical, as it is prohibitively expensive to implement in a consumer-grade digital TV receiver.
PN correlation-based channel estimation and known PN signal cancellation also have the disadvantage of only being updated at most once per frame. Consequently, when the channel characteristics are changed within a frame, known PN signal cancellation performance is undesirably degraded.
An adaptive noise cancellation is commonly used to remove noise from a signal, and is known to one skilled in the art. However, adaptive noise cancellation can be applied in a novel and unconventional way to known signal cancellation and channel estimation. In such an unconventional application, the output signal is used to update the coefficients of the adaptive filter, using well-known techniques like Least Mean Squares (LMS). In the LMS technique, coefficient updates are done once for every discrete time filter output, and therefore can be performed more than once per frame. In the unconventional application of noise cancellation, the known portion of the signal is treated like “noise” and the coefficients of the adaptive noise canceller represent the channel impulse response for the noise-cancelled signal.
Thus, the need arises for a receiver which includes an adaptive known signal canceller having channel estimation capability that is done inexpensively and with higher accuracy.